In a conventional ship such as a tanker and a transport vessel, an amount of electricity consumed by various auxiliary equipment, a cargo device, illumination, an air conditioner and other devices is huge, and to supply electricity to these electric devices, the ship includes a diesel power generator in which a diesel engine and a power generator for generating electricity by a driving operation the diesel engine are combined (see Patent Document 1 for example). It is known that the diesel engine is one of internal combustion engines having the highest energy efficiency, and an amount of carbon dioxide included in exhaust gas per unit output is small. Further, since the diesel engine can use low quality fuel such as heavy oil, the diesel engine has an advantage that it is economically excellent also.
In exhaust gas of the diesel engine, many nitrogen oxides, sulfur oxides and particulate materials are also included in addition to carbon dioxides. They are mainly produced from heavy oil which is fuel, and they are harmful materials which are obstructive to environment conservation. Especially nitrogen oxide (NOx, hereinafter) is harmful to human bodies, and exhibits strong acid, and it is considered that the nitrogen oxide is the cause of acid rain. Therefore, a machine which drives the diesel power generator such as a ship discharges an extremely large amount of NOx, and it is considered it largely damages the global environment.
As postprocessing means for largely purifying NOx, a selective catalyst reduction method (SCR method, hereinafter) using urea as a reducing agent is generally used. The SCR method uses a NOx catalyst of a honeycomb structure made of a material that oxide carrier such as Ti carries an active component such as V and Cr. If urea water as reducing agent aqueous solution is sprayed on an upstream side of the NOx catalyst, the urea water is hydrolyzed by heat of exhaust gas and ammonia is produced, the ammonia as a reducing agent acts on NOx, and NOx is decomposed into harmless nitrogen and water.